Stable Swirl Research Articles

Aeroacoustics of Drag-Generating Swirling Exhaust Flows

Aircraft on approach in high-drag and high-lift configuration create unsteady flow structures which inherently generate noise. For devices such as flaps, spoilers and the undercarriage there is a strong correlation between overall noise and drag such that, in the quest for quieter aircraft, one challenge is to generate drag at low noise levels. This paper presents a rigorous aero-acoustic assessment of a novel drag concept. The idea is that a swirling exhaust flow can yield a steady, and thus relatively quiet, streamwise vortex which is supported by a radial pressure gradient responsible for pressure drag. Flows with swirl are naturally limited by instabilities such as vortex breakdown. The paper presents a first aero-acoustic assessment of ram pressure driven swirling exhaust flows and their associated instabilities. The technical approach combines an in-depth aerodynamic analysis, plausibility arguments to qualitatively describe the nature of acoustic sources, and detailed, quantitative acoustic measurements using a medium aperture directional microphone array in combination with a previously established Deconvolution Approach for Mapping of Acoustic Sources (DAMAS). A model scale engine nacelle with stationary swirl vanes was designed and tested in the NASA Langley Quiet Flow Facility at a full-scale approach Mach number of 0.17. The analysis shows that the acoustic signature is comprised of quadrupole-type turbulent mixing noise of the swirling core flow and scattering noise from vane boundary layers and turbulent eddies of the burst vortex structure near sharp edges. The exposed edges are the nacelle and pylon trailing edge and the centerbody supporting the vanes. For the highest stable swirl angle setting a nacelle area based drag coefficient of 0.8 was achieved with a full-scale Overall Sound Pressure Level (OASPL) of about 40dBA at the ICAO approach certification point.

Experiments on a rotating-pipe swirl burner

Laser-Doppler measurements of mean velocity components and Reynolds stresses are reported in the near-field of a stable swirling flame of a newly designed natural-gas-fired experimental burner. The swirling motion is generated by the rotating outer pipe of the annular air passage, thus providing well defined inflow conditions which can be easily reproduced in computational and modelling studies. Prior to LDA measurements, burner stability characteristics were determined in terms of gas flow rate and pipe rotation speed. The measurements, corresponding to a stable, short, blue flame regime, show that the flow and combustion are dominated by a distinct, standing toroidal vortex, providing a hot core as source of stabilisation.

Sloshing motion of a magnetic fluid in a cylindrical container due to horizontal oscillation

Sloshing is a phenomenon that occurs when a liquid with a free surface is severely agitated in a liquid storage container. In an axisymmetric container, even though the excitation is lateral, the liquid oscillates with rotational movement around the central axis of the container. This phenomenon is called swirling and may be unstable swirling or stable swirling. We measure the velocity profile of a sloshing magnetic fluid using an ultrasonic Doppler velocimetry method called UVP, and examine the relationship between internal velocity profiles and swirling phenomena, in particular, stable swirling phenomena. We calculated power spectra from the velocity data using a fast Fourier transform and observed that the most dominant peak in the power spectrum moved to higher frequencies as the magnetic field intensity was increased, whereas a derived peak at a lower frequency moved to lower frequencies as the magnetic field intensity was increased.

Combustion Characteristics of a Premixed Test Combustor for a Hydrogen-Fueled Micro-Gas Turbine.

Lean premixed combustion may reduce NOx emission of gas turbines. On the basis of this concept, a micro gas turbine test combustor using a lean premixed swirling hydrogen flame has been developed. To attain experimental understanding of combustion characteristics of the combustor, stability limits, temperature and flow fields, and exhaust emissions were measured. The measurements showed that a stable swirling hydrogen flame attached to a flame holder was formed even in high-velocity premixed streams at low equivalence ratios. The combustor had the high rate of volumetric heat release of 2.5×103 W/(m3·Pa), and attained high combustion efficiencies of over 99.98%. Hydrogen combustion in the combustor resulted in extremely low NO-x emission levels of under 1 ppm, in comparison with a hydrogen-burning diffusion combustor.

Effect of Chamber Height on Centrifugal Separation Performance of a Gas-Liquid Vortex Chamber.

The effect of vortex chamber height on the characteristics of gas-liquid centrifugal separation in a vortex chamber has been investigated experimentally. It was made clear that the stable swirling liquid flow could be formed along the outer wall of the vortex chamber with the height of 20mm to 40mm. However, the unstable radial drop flow was observed in the height of 80mm over the range where the separated liquid fraction dropped steeply. In the shorter chamber, the radial width of separated liquid flow and the separated liquid fraction became larger. The data obtained here could be useful for designing a short vortex chamber.

Mixing of chemically reactive fluids by swirling in a tubular reactor

To achieve a varying degree of radial mixing in a single phase fluid, a stable laminar swirl is created by two fluid streams tangentially entering a tubular reactor. The details of the resultant fluid motion is described using the material stretching coordinate technique. The combination of fluid motion, molecular diffusion, and intrinsic chemical kinetics provides the basis for investigating mixing with chemical reactions for nonpremixed feeds. Computations on an infinitely fast second-order reaction for the radial Peclet number ranging from 10 2 to 10 6 reveal the following features for